The technique of multi-blade abrasive wire sawing has been in commercial use for several years. The process is used when it is required to slice thin plates or wafers from bars or blocks of expensive and fragile semiconductor or electro-optic materials.
In the standard process a high tensile steel wire of 150 to 180 microns diameter, which may or may not be coated with a layer of a softer material such as brass, is taken from a supply reel which can contain 200 km. or more of the wire, and passed under tension round a series of four motor driven wire guide rollers before being taken up on a collection reel. The surface of each of the wire guide rollers has a series of closely spaced ‘V’ shaped grooves machined in it at a pitch separation equal to the thickness of the wire plus the thickness of the required wafer. The lengths of the rollers are such that the wire can pass round them 4-500 times producing a four-sided flat loom or web of wires, where the wire is passing round the rollers at speeds of up to 10 meters per second.
This web may be flooded with a suspension or slurry of a finely divided abrasive powder such as 10 micron silicon carbide powder in a lubricating and cooling medium such as paraffin oil or polyethylene glycol. The material to be sliced is glued or fixed upon a mounting plate in the form of bars and these are then pressed against and slowly lowered through the upper and lower horizontal webs where the fast moving wires together with the abrasive slurry cut a regular series of thin slots through the material producing wafers or plates of accurate thickness and with a fine surface finish.
Two examples of wire sawing apparatus for carrying out such a process are shown in U.S. Pat. Nos. 5,564,409 and 5,616,065.
In the current industrial equipment the web is in the form of one single continuous array of wires and the width of the web is greater than the length of the material which is to be sliced. This arrangement is appropriate and cost effective when the whole of the material is of uniform and acceptable quality for the production of wafers or plates. However, this is not always the case, and in certain circumstances it is important that any region of the material which is not in a condition to produce commercially acceptable wafers, (because of defects in its chemical, physical, or electronic properties), is not sawn because this will convert up to 50% of the material into unrecoverable saw-dust which might otherwise be retained for subsequent recycling.
One example of such a circumstance occurs in the production of multicrystalline silicon wafers for use in photovoltaic (PV) industry. In this process silicon metal is melted and slowly cooled inside a crucible to form an ingot. The ingot is then sawn into a series of tall blocks of uniform square cross-section, which are then sliced into thin wafers in multi-bladed abrasive wire saws.
Because of the technical limitations on the heights of the silicon blocks which may be produced by the casting method, it is industrial practice to fix two or more blocks end to end upon the mounting plate as a single bar in order to fill up the maximum web width on the saw. Whilst this would appear to maximize machine capacity utilisation, it has been found to have considerable drawbacks.
The casting process used commercially to manufacture the silicon ingots results in the production of unacceptable quality material in both the bottom and top few centimeters of the blocks. This material is presently removed using a diamond faced circular or band saw before the blocks are mounted for wafering by gluing them onto a glass or ceramic mounting plate with their adjacent ends slightly separated or sometimes glued to each other.
This arrangement means that the web not only extends beyond both ends of the material to be cut, but also covers the centre section where the blocks butt up to each other or are glued together. It is inevitable that one or more wires in these positions will be cutting incomplete or thin wafers which have a high probability of breaking.
Wafers which break during the actual wire sawing process can have profound effects upon overall sawing yield. Not only can the broken wafers cause jamming of the slots and breakage of the wire, but pieces of broken wafer can be carried onto the surface of the wire roller guides where they cause damage to the precision grooves and result in irregular wafer thicknesses. In the high throughput sawing systems used in the PV industry where two parallel lines of blocks are cut at the same time upon a single web surface, broken wafer fragments can be transported to the down-stream blocks causing catastrophic wire breakages.
The present invention relates to an innovative modification of the process where the single web is separated into two or more webs by the introduction of sets of adjustable angled pulleys into the wire loom.